Conventionally, there is a known technique of forming a plurality of annular grooves in a sliding face of a seal lip in a sealing device for reciprocation in order to reduce frictional resistance. Moreover, in order to suppress reduction in a sealing property due to the plurality of annular grooves formed in this manner, there is a known technique of forming opposite side faces of each of the respective annular grooves at different inclination angles with respect to the sliding face to return sealed fluid such as oil to a sealed fluid side (see Patent Literature 1). This is a technique in which an edge between the side face on an opposite side from the sealed fluid side and the sliding face in the annular groove is formed to have a smaller angle than an edge between the side face on the sealed fluid side and the sliding face. With this technique, a returned amount of the sealed fluid to the sealed fluid side becomes larger than a leakage amount and it is possible to suppress reduction in the sealing property.
Here, in a case of sealing oil at hydraulic pressure between 1 MPa and 2 MPa and if the oil is oil of poor lubricity (low viscosity) such as brake oil, for example, a lip tip end of a seal lip in a sealing device is formed by a curved face and rubber material of relatively low hardness (hardness of about 70 degrees) is employed as rubber material for the sealing device made of rubber, in general.
If the above-described prior art is applied to this case in order to reduce frictional resistance, the edge is formed between the side face of the annular groove and the sliding face and therefore a lubricant oil film is scraped off cleanly and it is difficult to obtain a lubricating effect. There are other related techniques as disclosed in Patent Literatures 2 and 3.